Program Speaker
Scott Wilson: How to make a calcite dichroscope.

by Scott Wilson and Nancy Attaway

Scott Wilson’s presentation centered around making and using a calcite dichroscope. Scott explained how to make a simple dichroscope with a crystal of calcite, plastic tubing, and a small lens. He explained that a dichroscope was used in gem identification to determine pleochroism in gems, a property of certain gems where more than one color is shown when the gem crystal is viewed in various directions.

Scott first explained pleochroism as the differential spectral absorption of polarized light in colored gem crystals, and he remarked that pleochroism occurs only in crystals. Pleochroic gems that crystallize in the orthorhombic, monoclinic, and triclinic crystal systems may show three colors when viewed in different directions, but one of those colors may be pale or very faint. Gems of the tetragonal and the hexagonal crystal systems show two colors when viewed in different directions. Pleochroism never occurs in glass (or in opal), in colorless crystals, in plastic or in any isometric (cubic) crystal. Scott said that pleochroism can be difficult to characterize in polycrystalline materials, like agate, because such materials scatter light instead of transmitting light.

Scott explained how light, transmitted through doubly refractive gems, vibrate in two planes at right angles. The two beams of light then undergo an unequal reduction in velocity, and, as a result, the two beams undergo unequal absorption in anisotropic gems to emerge as different colors. Those gems that show three different colors are said to be trichroic. Gems that show two distinct colors are said to be dichroic. Pleochroism can sometimes be seen with the naked eye, but a dichroscope allows us to see more than one color in any single direction. The measure of the ability of a gem to convert a single ray of light into two rays having unequal velocity is known as birefringence.

Scott said that a dichroscope identifies glass immediately. He said that many of the more expensive gems are not cubic, with the exceptions of diamond, garnet, and spinel. Scott remarked that some gems may show a very weak pleochroism. The dichroscope can help orientate gem rough for faceting, where no color change is seen, when it is aligned with the crystal axis. A dichroscope can also show the C axis, and with gems like peridot, the dichroscope helps to exhibit the best color when the table is oriented down the C axis. Scott said that the dichroscope proves double refraction if seen, but it does not prove the absence of double refraction if it is not seen. The dichroscope can also separate corundum from spinel (red and blue).

A dichroscope may be priced between $125 and $175, on the average, but Scott explained how a dichroscope can be made for under three dollars. The early models for dichroscopes used transparent Iceland spar calcite to separate the two colors seen in pleochroic gems in a direction other than parallel to an optic axis. Other types of dichroscopes used two pieces of Polaroid film set with their transmission directions at right angles. Scott said that when light enters the calcite, light is broken into two polarized rays that have vibration directions at right angles to each other. These rays are slowed down unequally by the calcite and are bent or refracted, one more than the other. Two images of a square aperture are visible through the dichroscope, and the images will show two different colors when a pleochroic gem is seen through a dichroscope at various directions. In the dichroscope with Polaroids, the colors seen through the Polaroids set at right angles will be different.

Scott’s homemade dichroscope was composed of a PVC end cap that costs $0.30, a PVC coupling that costs $0.50, a clear calcite rhombohedron crystal that costs $1.00, and a cheap loupe that costs $1.50. Scott’s instructions on building a dichroscope began with obtaining a crystal of water-clear calcite, one with no inclusions or colors due to twinning or fractures. He said to polish the ends, which may take some time. Some faceters use a wax lap to polish calcite. Then, get a cheap loupe. Buy a PVC fitting at a hardware store that the calcite crystal will almost fit into and also buy a PVC coupling that will fit that and the loupe. Use a file to cut some groves in the fitting to hold the calcite crystal snugly. Punch a hole in the center of the fitting. He said that a hot, square piece of metal about 1/16 of an inch in diameter was a good start. Assemble the calcite crystal into the fitting, the fitting into the coupling, the loupe into the coupling, then look through it. Widen the square hold until the two images just barely touch. Paint the inside black if you want to reduce the reflections. Hold it all together with glue, tape, or friction.

Scott then explained how to use the dichroscope. Look through it at a stone that is illuminated in transmission (a tungsten light source is usual). Rotate the dichroscope around its axis while looking for a change in color. Note when the color is strongest and which colors or shades that you see. Rotate the stone at various orientations and then repeat the process.
Scott said that when the colors change, the material is doubly refracting and, therefore, is not isometric (cubic) or amorphous (glass). When the colors do not change in one direction of the material, you are along an optical axis. When only two colors are seen, the material is likely to be uniaxial (tetragonal or hexagonal). When three colors are seen, the material is likely to be biaxial (orthorhombic, monoclinic, or triclinic). Scott also said to look for special effects of the light source, as fluorescent lights are bad for giving those.

Scott remarked that the size of the calcite crystal would determine the size of the square seen through the dichroscope. A long calcite crystal would show a small square, and a short calcite crystal would show a large square. Scott said that a square hole allows you to rotate 90 degrees easier than a rounder hole.

Scott warned that colors may show such subtle differences that you might think that they are the same, which is common in biaxial materials. Color differences may be stronger in one part of a crystal than in another, possibly due to strain, twinning, or color zoning. The tables of dichroic colors describing a gemstone may state something like: “pale blue-green/ pale yellow-green to colorless”. That leaves room for interpretation and may be confusing. The reference may not identify the light source as natural (north daylight) or artificial (tungsten, most likely).

Scott concluded his talk by providing several sources for more information. He recommended the following references: Gem Identification Made Easy by Matlins and Bonanno for tables of dichroic colors for common gem materials; Gemology by Hurlbut and Kammerling for data on special properties; Mineralogy by Sinkankas for more data and very good explanations of crystal relationships; and Handbook of Gem Identification by Liddicoat for data, definitions, and tables. He then passed around his homemade dichroscope with a gem crystal tourmaline and a light source to show members how it works.

Thanks to Scott for a good lesson in making a practical gem identification tool from simple objects. We can actually do this one at home!